Conventional micro-electro-mechanical (MEMs) micro-mirror devices 1, illustrated in FIG. 1, comprise a mirror platform 2, suspended above a substrate 3 via hinges 4 and cap 5. One or more pedestals 7, extend upwardly from the substrate 3 in the middle or on each side of the mirror platform 2 for supporting ends of the hinges 4. A reflective coating 6 (typically metallic) is disposed on the top surface of the mirror platform 2 for redirecting optical signals in dependence upon the tilt angle of the mirror platform 2 relative to the substrate 3. Residual stresses in the reflective coating 6 introduces a mirror curvature in the mirror platform 2, which adversely affects optical performance. Moreover, any change in the curvature of the mirror platform 2, e.g. due to metal stress relaxation, affects reliability. Therefore, stiffening the mirror platform 2 is highly desirable to control stress-induced mirror curvature.
Typically, as illustrated in FIG. 1, the mirror platform 2, the hinge 4 and the cap 5 have a uniform thickness, whereby the aforementioned structures can be formed, e.g. etched, in a single etching step, and mounted on the raised pedestal 7 extending from the substrate 3. Hot electrodes 8 are positioned beneath each side of the mirror platform 2 for selectively attracting the underside of the mirror platform 2, which act as ground electrodes, for tilting the mirror platform 2, as desired, e.g. for switching optical signals. The tolerance of the thickness of the mirror platform/hinge affects the stiffness of the hinge 4, and also affects the electrode gap 9 between the hot electrode 8 and ground electrode, i.e. the underside of the mirror platform 2. The tolerance of the electrode gap 9 determines the variation of the driving torque generated for a given voltage applied to the hot electrode 8. The advantage of the design shown in FIG. 1 is a self-compensating effect from the etch depth, whereby an increase in hinge stiffness due to a smaller etch depth, i.e. a thicker hinge 4, is compensated by an increase in the electrostatic force from a smaller electrode gap 9, as a result of the smaller etch depth. The etch depth compensation reduces the voltage variation of the device due to etch depth tolerance. Unfortunately, there are several drawbacks to the conventional structure, which include:
1) A limited scope to improve the stiffness of the mirror platform 2; i.e. since the mirror platform 2 has a uniform rectangular shape, only the thickness of the mirror platform 2 can he adjusted to improve the mass moment of inertia (MOT) thereof, and the mechanical resonance thereof.
2) The process considerations for the width of the hinge 4 puts an upper limit on the thickness of the mirror platform 2, i.e. the stiffness of the mirror platform 2, since the hinge 4 has the same thickness as the mirror platform 2.
3) The mirror swing space for tilt is determined by the electrode gap, thereby constraining the electrode design by the swing space requirement and vice versa.
An object of the present invention is to overcome the shortcomings of the prior art by providing a micro-mirror structure that provides additional mirror stiffness, while maintaining the compensation of hinge stiffness by electrode gap, by means of an electrode spacer.